1. Field of the Invention
The present invention relates in general to an apparatus for recording and reproducing the information carried on a recording disc such as, for example, a video or audio disc to be optically, electrostatically or otherwise scanned. Particularly, the present invention is concerned with a signal processing circuit in a recording-disc reading and reproducing apparatus.
In a recording-disc information reading and reproducing apparatus of, for example, the optically or electrostatically scanning type, multiplex signals consisting of frequency modulated video and/or audio carriers superposed on each other are stored in the form of a series of depressed areas or "pits" formed in each or one information-carrying face of a video or audio disc. These depressed areas or pits are arranged in a spiral track or in a number of concentric tracks about the center axis of the disc. The video and/or audio information thus stored in the recording disc is read out by optically, electrostatically or otherwise scanning the individual pits along the spiral track or each of the concentric tracks. In the case of an optically scanning video or audio information reading and reproducing apparatus, for example, the video and/or audio information stored on a video or audio recording disc is read out by scanning the pits by means of a beam of laser light and thereby detecting the lengths of and spacings between the scanned pits. During playback of such a recording disc or during selection of desired pieces of information out of the information carried on the information-carrying face of the disc to be played back, the disc is driven for rotation about the center axis thereof and the beam of the laser light is displaced radially of the information-carrying face of the rotating disc. The laser beam thus directed onto a target track on the information-carrying face of the disc is reflected from the face or passed through the disc and the information picked up by the laser beam from the information-carrying face of the disc is converted into electric signals. These electric signals are further converted upon frequency demodulation into video and/or audio signals to be reproduced.
The laser beam to read out the information recorded on an information-carrying face of a video or audio disc is transmitted from a laser emitter to a tracking mirror through a lens system and is reflected from the tracking mirror toward the information-carrying face of the disc to be played back or scanned by the beam. The tracking mirror is turnably mounted on a slider movable back and forth in a radial direction of the disc to be played back or scanned and is urged to stay in a predetermined neutral or home angular position about the axis of rotation thereof on the slicer. During scanning of the video or audio disc to be played back, the slider is driven to travel in such a direction with respect to the disc and, concurrently, the tracking mirror is driven to turn between two opposite limit angular positions from the neutral or home angular position thereof about the axis of rotation of the mirror on the slider. These motions of the slider and the tracking mirror are effected under the control of a tracking servo system. The tracking servo system is arranged so that the servo loop to control the oscillating motions of the tracking mirror is closed when or after the laser beam directed toward the target track of the disc is incident on a spot close to the target track. If the servo loop is closed and the servo system is locked in before the scanning spot of the light is moved close to the target track, it may happen that the tracking mirror forming part of the optical pick-up system and operative to deflect the scanning beam in a radial direction of the video or audio disc is abruptly initiated into motion to reach the target track and thus overshoots the target track. In an extreme case, the tracking mirror may be caused to oscillate and disable the tracking servo system from being locked in. Such an event may be caused not only during scanning of a video disc but generally when the servo loop of the tracking servo system is to be closed from an open condition.
When the tracking mirror being turned about the axis of rotation thereof reaches one of the predetermined limit angular positions thereof, the tracking servo system controls the servo loop for the tracking mirror to open so as to allow the mirror to return to the neutral or home angular position thereof by the action of, for example, a return spring connected to or otherwise engaging the mirror. When the tracking mirror is thus returned to the neutral or home angular position thereof, the tracking servo system controls the servo loop for the mirror to close for a second time for enabling the mirror to repeat the oscillating motions.
The signals which are read out from an information-carrying face of a video or audio disc contain not only the video and/or audio signals to be reproduced but various control data signals such as the address signals indicative of the prescribed addresses in the spiral track or each of the concentric tracks on the information-carrying face of the disc. If there control data signals are used for the control of the movement of the mirror-carrying slider while the servo loop for the tracking mirror is open, the slider may be controlled erroneously and may fail to correctly scan the track or tracks on the information-carrying face of the disc being scanned by the beam.
The present invention contemplates provision of an improved tracking servo system which is free from such a problem.